1) airframe wear n tear
2) run out of funds to keep them flying post Herrick drawdown
3) RAF run short of regular serving pilots as the Reserve Pilots actually standdown.

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They'll be mothballed first. That's if they don't spend ten years in a hanger somewhere because the knobs on the radar are Bakelite and not melamine. Replacements are planned For about 200 years from now. This will mean reduced flying hours per year, but this is in keeping with current defence cost reduction planning.